Color television



A. c. scHRoEDER coLoR TELEVISION Original Filed Nov. 2, 1953 July 26,1960 INVENTOR .4L nego 5r/wann United States Patent" COLOR TELEVISIONAlfred C. Schroeder, Southampton, Pa., assignor to Radio Corporation ofAmerica, a corporation of Delaware 6 Claims. (Cl. 178-5.'4)

Thisapplication is a division of an application of Alfred C- Schroeder,iiled November 2, 1953, Serial No. 389,566, entitled Color Television. YThe present invention relates to color television systems of thesimultaneous subcarrier type, and more particularly to apparatus forimproving the ldetail rendition in color television systems of thistype. v In a color television system Which accords with the signalspeciiications proposed by the National Television Systems Committee(NTSC) to the FCC for adoption as color television signal standards,color information is conveyed through the medium of a modulatedsubcarrier wave. In the proposed system,.for color information in alimited low frequency band, the color subcarrier is both phase andamplitude modulated to permit effective threecolor reproduction ofrelatively large picture areas. For

rcolor information in a succeeding higher frequency band,

' rier which is modulated with so-called color-difference informationideally carries only chrominance information and has no effect on theluminanceof the `reproduction. In a perfectly linear system,it liscorrect` to say thatthe broad band signal provided for in the signalspecifications conveys -all the luminance information and that the'chrominance signals aifect only the chromaticity'. `How-- ever in a,system in whichthe respective component color signals are madenon-linear for gradientcofrrection pur- 2,946,846 Patented uly 26, 1960formation throughout the entirerange of amplitudes for the luminance andchrominance signals. However a more complex receiver would fbe requiredto obtain the properly related voltages for the steps involving thecombining of the luminance information and the color-differenceinformation to obtain the proper simultaneous color signals. Thus, sincethe signal specifications incorporate the latter manner of formation ofthe luminance signal, it must be recognized that the chrominance signalsdo havev some elfect on the luminance of the color reproduction. It maybe demonstrated that the contribution of the chrominance channel to theluminance of the reproduction increases With saturation of the subjectcolors and therefore is greatest in what may be referred to as highchroma areas of the picture. v v While it thus may be readilyappreciated that the desirable features of constant luminance operationare not fully realized in high chroma areas, a further problem residesin the fact that detail rendition in high chroma areas is also impaired.To more fully appreciate the reasons for this accompanying result, anexamination of the makeup of the chrominance signal should now be made.The chrominance` signal comprises the sum of two orthogonal componentsof a subcarrier Wave, respectively modulated by so-called I and Qcolor-mixture signals. One component of the color subcarrier of apredetermined phase relative to a reference phase is modulated inamplitude by the narrow band (approximately 500 kc.)EQ signal, which isequal to` 0.41 (EB-@+0.48 (ER-EY) and a second component of thesubcarrier in phase quadrature with the first is modulated in amplitudeby the moderately Wide band (approximately 1.5 mc.) EI signal, which isequal to i l With the color subcarrier frequency at 3.579545 mc., doublesideband transmission, reception and utilization of both color mixturesignals, EQ and EI, up to a 500 kc; limit,.is feasible to permit theaforementioned threecolor .reproduction of relatively large pictureareas without crosstalk, While in therband of signals from approximately500v kc. to 1.5 mc. where inherent system limitations dictateyeffectively single sideband transmission the 1 single color mixturesignal E; is utilized for the aforeposes andthe broad band signalYcomprises the sumof appropriate portions of these individually correctedcomponent ,colori signals, the chronn'nance signals; do :have someeifect on the luminance-so that: the broad band .signal mayno longer besaid to convey all luminance inforcorrected combination VVof the red,green and blue sig- J nals,-but rather a combinationofthe individuallygamma-corrcted red, green and blneysignals. 'Ifthe transmitted luminancesignalEY were made upfin theformer manner rather .than the latter, itmight moreproperlyibe that.the signal represents yalloftheluminancepinmethods, :andapparatus therefor, for' correcting" 'ths'd-'mentioned two-color reproduction of relatively small color-details,However, for the finest picture ldetail represented by signalfrequencies above approximately 1.5 mc., no attempt is made to conveychrominance information,.theserdetails being satisfactorily reproducedinV black-and-white only in accordancev with the well knownmixed highsprinciple. y '-Thus,l for-very tine picture detail thekreceivingappaatus can utilize only the information supplied by -th'e broad bandluminance channel. If the luminance channel truly conveyed all theluminance information as would bepreadily` possibleina linear system,the' very line Vdetail. rendition-in the color reproduction would beequal tQ that obtainedlinconventional black-and-White reproductions;Howeven as was noted previously, due to the. necessity "for correctingthe respective componentcolor signals for system non-linearities ,anddue. to the :fact- -that the luminance signalA comprises anappropriately pro'-y portioned combination of theseindividuallycorrected sig-4, nals,` all the luminance'information doesnot appearrlin, the luminance channel, particularlyin high-chroma areas.A result necessarilyis that detail* in`high"chroma regionsE is deficientin its luminanceA component. ,f-

The present inventiontherefore is directed tol novel iiciency byemphasizing the highs of the monochrome signal portion of the compositecolor picture signal in high chroma areas. In accordance with anembodiment of the invention, the high frequency components of themonochrome signal are obtained from a network which continually selectsthe component color signal of the greatest amplitude.

Another factor which contributes to loss of resolution in high chromaareas in reproductions of color television signals in a simultaneoussubcarrier type of system is the partial -rectication by the reproducingkinescope of subcarrier components reaching the kinescope. It will bereadily `appreciated vthat Vemphasis of the highs in high chroma areasin accordance with the invention serves also to compensate for thisadditional cause of loss of resolution in such areas. It should also benoted that the improvements in detail rendition which are attributed touse of the present invention are to be observed in `black-andwhitereproductions of color television signals as well as color reproductionsthereof.

Accordingly it is a primary object of the present invention to provide acolor television system of the simultaneous subcarrier type withapparatus for improving detail rendition in color and black-and-whitereproductions.

`It is a further object of the present invention to provide novelapparatus for forming a luminance signal in a color television system ofthe simultaneous subcarrier type.

It is an additional object of the present invention to provide a colortelevision system with improved detail rendition in highly saturatedpicture areas.

It is another object of the present invention to provide a novel systemof high peaking in the luminance channel of a color television system.Other objects and advantages of the present invention will becomereadily apparent to those skilled in the art upon a reading of thefollowing detailed description and an inspection of the accompanyingdrawing in which:

The single figure illustrates in block and schematic form a colortelevision transmitting system in which the monochrome portion of thecomposite color picture signal is formed in accordance with anembodiment of the present invention.

Referring to this gure in greater detail, a camera 11 is illustrated asthe source of three simultaneous component color signals ER, EG and EB.The individual component color signals are applied to respectivegradient correction amplifiers or so-called gamma correction ampliiiers13, 15 and 17. The respective output signals are representative of theoriginal componentcolor signals after introduction of a compensatorynon-linearity, which in conjunction with camera non-linearity provideswhat is essentially the complement of the ultimate image reproducersnon-linearity.

As in color television systems of the type described in the articlePrinciples and Development of Color Television Systems by G. H. Brownand D. G. C. Luck, appearing in the June 1953 issue of the RCA Review,the individually gamma-corrected color signals .may be applied tosuitable matrixing a circuit 19, wherein the respective color signalsmay be combined in appropriate proportions and suitable polarities toobtain the desired color-mixture signal ouputs. In a color televisionsystem of thetype disclosed in the aforementioned article and which isin accordance with the aforementioned NTSC signal specifications, thecolor-mixture signal outputs may comprise so-called Q and I signals ofthe following character: Y

. EQ=O.21ER0.31EB and EI=0.60ER0.28EG+0.32EB I n further accordance withsuch proposed systems the Q and I signals may be passed throughrespective low pass lters 21 and 23 having passbands of 0 to 0.5 mc. and0 to l.5 mc. respectively, and applied to respective subcarriermodulators 25 and 27. The Q signal modulates waves of subcarrierfrequency supplied by the subcarrier source 29 having a predeterminedphase relationship relative to some reference phase, while the I signalmodulates waves of subcarrier frequency from source 29 which are inphase quadrature with those subject to the Q signal. Respective bandpassfilters 31 and 33 are provided to select from the modulation products ofthe respective modulators 25 and 27, a narrow band, double sidebandsignal lying in the 3 to 4.2 mc. band and a wider band, partially singlesideband signal lying in the 2 to V4.2 mc. band.

A sync signal generator 35 is conventionally provided to synchronouslycontrol the generation of scanning waves for the camera 11 in the cameradeiiection wave generators 37. The sync signal generator 35 is tied tothe subcarrier source 29 to insure that the desired relationship betweenscanning and subcarrier frequencies is maintained, which tie-inrelationship is generally provided by deriving the scanningsynchronization signals from the subcarrer source output by suitablefrequency division.

An adder 39 is provided for combining the Q and I modulated subcarrierwaves and the appropriate synchronizing signals derived from the syncgenerator 3S and subcarrier source 29 with a broad band monochromesignal. The formation of a monochrome signal for combination with theaforementioned signals in a color television system of the typedescribed is the particular subject of the present invention and willnow be described in detail. n

In addition to application of the individually gammacorrected colorsignals to a matrixing circuit 19 for formation of the Q and I colormixture signals, the individually gamma-corrected color signals are alsoapplied to an additional matrixing circuit d@ for combination thereof toobtain a so-called luminance signal EY, nominally representative of theluminance of the subject image elements. Inraccordance with theaforementioned NTSC signal specication and with the constant luminancefeature incorporated therein, the component color signals are combinedin proportion to the relative luminosities of the respective primaries,i.e.

It has heretofore been general practice to utilize a signal soproportioned as the entire broad-band monochrome portion of thecomposite transmitted color picture signal.

Thus the output of the matrixing circuit 40 would normally be passedthrough a low pass lter having a passband of 0 to `approximately 4.2 mc.and combined with the other signal components in adder 39. However inaccordance with the present invention, whereby as previously indicatedit is desired to compensate for detail loss in high chroma areas of thepicture, a Y signal so proportioned is utilized only as the lowfrequency component of the transmitted monochrome signal. 'Ihus the lowpass lter 41 having a passband of 0 to 1.5 mc. is provided in the Ysignal path between the matrixing circuit 40 and adder 39, so that theluminosity-function proportioning of the monochrome signal makeupapplies only inthe range of frequencies occupied by one or both of the Iand Q signals. The high frequency component of the monochrome signal isseparately formed 1n a novel manner.

In accordance with the illustrated embodiment, a third matrixmgnetwork43 is provided for the individually gamma-corrected color signalsin addition to the previously discussed matrixing circuits 19 and 40.The matrixing network 43 is included to provide a plurality of outputsignals, representative of equal proportions Yof therespectivercomponent color signals, or different proportions thereof, orrepresentative of predetermined mixtures thereof, which output signalsmay be applied to a signal comparison network 45 from comparison inamplitude and selection of the respective signal of greatest amplitudein accordance with the principles of the invention to be subsequentlydiscussed. Desirable improvements in detail rendition have been achievedin accordance with the invention where the signals applied to thenetwork 45 were respectively representative of the three individuallycorrected component color signals, without different relativeadjustments in gain therefor and without mixing of portions thereof. Insuch instances, the so-called matrixing network 43 needs to perform nomatrixing function but may serve only to provide equal gain amplifiersfor the respective component color signals, or may even be omitted.However, it appears desirable to include such apparatus as the matrixingnetwork 43 in the coupling of the component color signals to thecomparison network 45, so that should image conditions, systemlimitations or other circumstances so indicate, relative adjustments ofthe respective component color signal gains or actual matrixing of thecomponent color signals to provide predetermined color mixture signalsmay be achieved to avoid excessive detail-loss compensation orinsuicient detail-loss compensation for particular color changes orunder particular brightness conditions. For the purposes of thefollowing description, however, whereby one may most readily arrive atan understanding of the principles of the invention it will be assumedthat the matrixing network 43 supplies the comparison network 45 withthree respective signals substantially corresponding to therindividuallycorrected component color signals without different relative gainadjustments and without matrix formation of mixture signals. j Y n VInaccordance with the invention the respective color signal outputs 'ofthe matrixing network 43 are applied to the signalV comparison network45 having an output terminal 77, the network 45 operating to compare inamplitude the respective color signals, and to continually select andpasstovthe output terminal the color signal of greatest amplitude. Whileconceivably other apparatus may be jderivedto accomplish this function,the illustrated embodiment utilizesa network quite similar tothatshownin my U.S. Patent 2,646,463, tiled July 18, 1951, andu entitledApparatus for Reproducing Images in Color, wherein apparatus wasprovided for continually selecting the component color Vsignal ofsmallest amplitude, for purposes of an entirely different nature.

Thus, the respective color signals are applied to conventional D.C.restorers or level Setters 511, 53 and 55 so that any D.C. low frequencycomponents which may have been lost in the preceding stages may berestored. The output signals of the level setters 51, 53 and 55 areapplied `to respective cathode followers 57, 59 and 61. The cathodes ofthe cathode followers 57, 59 and 61 are coupled to a common junction,the output terminal 77, by similarly polarized diodes (thermionic orcrystal) 71, 73 and 75 respectively. With thepolarity of the signalsapplied to the cathode followers being such that the respective cathodesthereof become more positive with increase in color signal intensity,the polarity of the diodes is chosen such that the plates of the diodesare connected to the cathodes of the corresponding cathode followers.

The selection of the color signal having the highest amplitude may beexplained as follows. With no color signals present, the diodes allconduct to the same degree. As the intensity of a color signalincreases, the potential applied to the plate of the corresponding diodebecomes more positive. The diode to which the largest color signal isapplied conducts the most and the potential of all the diode cathodesdrops to a value just below that of the largest color signal. Thislatter value is greater than the other color signals and therefore theother color 6 v signals cannot pa'ss to the common junction, outputterminal 77.A

Thus it will be appreciated that the signal appearing at the outputterminal 7'7' is continuously representative of whichever color signalVis greatest at any instant. The result is that, in high chroma areaswhere the amplitude of one colory signal may be substantially greaterthan the amplitude of one or both of -the other color signals, theoutput at terminal 77 corresponds to thatdominant color signal ratherthan corresponding to a mixture of the three signals. The output signalsappearing at terminal 77 are passed through a high pass filter 81,having a passband encompassing the 1.5 to 4.2 mc. high frequency portionof the broad band generally assigned to the monochrome signal. Theoutputs of high pass ilter 81 and low passV lter 41 are combined inadder 39 to provide the full 4.2 mc. monochrome signal, which inaccordance withthe invention comprises a low frequency portion (suppliedthrough filter 41) which is the luminosity-function mixture of the colorsignals desired for constant luminance, operation, and a high frequencyportion which corresponds as previously indicated to the color signal ofgreatest amplitude.

TheV improvement in detail rendition in high chroma areas when themonochrome signal is made up in such a manner may now bev more readilyappreciated. As an example, let us consider the signals derived from apicture area which is predominantly a saturated blue. As has beenpreviously noted, in high chroma areas, such as the saturated blue areachosen for illustration, some of the luminance information concerningthis area necessarily appearsfinjthe chrominance channel. For signals upto 1.5 mc. this is not a problem insofar as loss of information is`concerned since, at least for the I signal, the chrominance channelpasses signal frequencies upto -this limit.I For signal frequenciesabove 1:5 mc. however, the luminance information contained in thechrominance channel is effectively lost. If the monochrome signalin `therange of signal frequencies above 1.5 mc. is still made up of theluminosity-function mixture of color signals, the high frequencyluminance information containedtherein may be seriously deficient. If,on the other hand, the high frequency components of the Ymonochromesignal always correspond to the colorY signal of greatest amplitude, thedeficiency is substantially avoided. T-hus, in the illustrativesaturated blue area, the high frequency components of the monochromesignal would essentially comprise the high amplitude output of the bluesignal source rather than a mixture signal made up of approximately 11percent of the high amplitude blue signal, 30 percent of the lowamplitude red signal and 59 percent of the low amplitude green signal.

Thus it will be seen that for high chroma areas, such as the,illustrative saturated blue area, the net -result of practicing thepresent invention is an effective peaking of the highs in the monochromesignal to make up for the loss of high frequency luminance informationin the chrominance channel. However, it may be noted that in white orgray image areas where the respective cornponent color signals areequal, the output of the signal comparison network 45 will beessentially the same as the output of the luminosity-functionproportioning matrix 40. Use of the present invention thus involves nochange in the appearance of the monochrome signal for white or grayimage areas. Since, however, there is no loss of luminance informationin the chrominance channel for white or gray image areas, the absence ofeffective peaking of the highs for white or gray signals is a desiredresult of the use of the invention. Also, it may be noted that incolored areas of low saturation, where the respective component colorsignals are nearly equal, the high frequency signals selected `from theoutput of the signal comparison network 45 will be only slightly greaterin amplitude than the high frequency signals which would otherwise beselected from the output of the matrix 40. Thus, the `degree of electivepeaking of the highs 'in the monochrome signal formed in accordance withthe invention is observed to decrease with decrease in saturation of theimage colors. This is also a desired Aresult of use of the presentinvention, since as Vhas been previously indicated the deficiency ofluminance information in the monochrome signal for which it is desiredto compensate increases with saturation.

I claim: Y

1. In a color television system including a source of a plurality ofsimultaneous component color signals, apparatus comprising thecombination of means coupled to said source for combining all of saidcomponent color signals in relative proportionssubstantiallyrcorresponding to the relative luminosities of saidcomponent colors, a low pass lter coupled to said component color signalcombining means, additional means coupled to said source and having anoutput terminal for continually selecting and passing to said outputterminal the component color signal of greatest magnitude, a high passfilter coupled to said output terminal, and signal adding means coupledto said low pass and high pass iilters.

2. In a color television system including a source of a plurality ofsimultaneous component color signals, apparatus comprising thecombination of a matrixing circuit coupled to said source for forming asignal nominally representative ofthe luminance of the subject imagefrom said plurality of simultaneous component color signals, signalcomparing means also coupled to said source for forming a signalcontinually representative of the component color signal of greatestamplitude, low pass iilter means coupled to said matrixing circuit, highpass lter means coupled to said signal comparing means, and means forcombining the outputs of said low pass and high pass tilter means.

i 3. In a color television system of the simultaneous subcarrier typewherein the transmitted `component color picture signal includes abroadband monochrome signal, said system including a source of a pluralityofrsimultaneous component color signals, apparatus comprising thecombination of a matrixing circuit coupled to said source for combiningall of said component color signals in relative proportionssubstantially corresponding to the relative luminosities of saidcomponent colors to form a signal nominally representative of theluminance of the subject image, signal comparison means also coupled tosaid source for developingr a signal continually repre- `sentative ofthe component color signal of greatest amplitude, a pair of filtermeans, one of said pair of lter means being coupled to said matrixingcircuit and having a passband encompassing a low frequency portion ofsaid broad band, the other of said pair of filter means being coupled tosaid signal comparison means and having a passband encompassing theremaining high frequency portion of said broad band, and means forcombining the outputs of said pair of lilter means to form said broadband monochrome signal. A

4. Apparatus in accordance with claim 3 wherein said signal comparingmeans includes an output terminal, a plurality of signal handlingdevices, each of said component color signals being applied to arespective one of said plurality of signal handling devices and aplurality of diodes, each of said diodes being coupled between arespective one of said signal handling devices and said output terminal.l

5. In a color television system ofthe simultaneous subcarrier typeprovided with a plurality of simultaneous component color signals,apparatus for developing a broad band monochrome signal comprising thecombination of means for matrixing said plurality of component colorsignals to provide a iirst monochrome signal, additional means -formatrixing said plurality of component color signals to provide aplurality of respectively different color signals, means for selectingthe signal of greatest amplitude from said latter plurality of colorsignals, and means for lcombining high frequency components of saidselected signal with low frequency components of said iirst signal toform said broad band monochrome signal.

6. Apparatus in accordance with claim 5 wherein each of said latterplurality of color signals substantiallycorresponds to a respective oneof said plurality of simul y taneous component color signals.

References Cited in the le of thispatent UNITED STATES PATENTS 2,646,463Schroeder July 21, 1953 2,748,190 Yule 1-1- May 29, 1956 2,841,640Bartelink July 1, 19.58

